The present invention relates to a synchronous motor having a coil fastened at the inner side of a stator and a rotor carrying coil soft iron pole rings adjacent both sides of the coil, the pole rings being joined rigidly with a core of high permeability passing through the center of the coil, the pole rings facing permanent magnet poles of the stator.
Such synchronous of motors, also called stepping motors, generally are already known; e.g., from the West German printed specification No. 29 12 688 (FIG. 14) or from IBM Technical Disclosure Bulletin, vol. 9, no. 7, December 1966. The rotor of such motors consists of relatively light parts, so that it exhibits a small moment of inertia with respect to the driving moments acting upon it. This permits motors of relatively high power to be built with conventional dimensions. However, the high driving moments combined with the small moment of inertia of the rotor gives rise in its turn to instabilities at the start and even during the normal running of the motor.
It is therefore a feature of the present invention to construct a synchronous motor like the one described above with a minimum of expenditure of material and space, and capable of reliable and stable running in all conditions of operation.